1. Field of the Invention
The present invention relates to measuring and evaluating the particle size distribution of particles in aerosol and, in particular, can quickly and easily measure and evaluate particles having a particle size not more than 100 nm and thus is suitable for in-situ measuring particles in a reduced pressure vapor phase process apparatus and a clean room used for manufacturing a semiconductor integrated device and a liquid crystal display device and contributes to the improvement of the manufacturing yield of these devices.
2. Description of the Related Art
As an example in the related art, a particle size measuring unit using a laser scattering method, which is a mainstream at present. This is a method of measuring the particle sizes of particles in aerosol and utilizes the phenomenon that when laser light is applied to the aerosol, the spatial intensity distribution of diffracted light of the laser is varied by the distribution of particle size. The constitution and operation to be described below with reference to FIG. 1 are now widely used in the industry concerned, and is disclosed in a literature, for example, “Particle Size Measuring Technology” compiled by Society of Powder Technology, Japan, published by Daily Industry Newspaper Co. Ltd., (1994), Item 145 to Item 148.
A helium-neon (He—Ne) or semiconductor probe laser 1 having an output power of several mW is used as a light source. Its luminous flux is expanded into a parallel luminous flux having a diameter of several mm by a beam expander 2 and is applied to a group of particles 3 in aerosol introduced into a measuring section. In this beam expander 2 is built a spatial filter so as to produce irradiating luminous flux having high parallelism. The laser light scattered by the group of particles in the aerosol is refracted by a receiving lens 4 and is entered into a detector 6 on a focal plane 5. A fθ lens is used as the light receiving lens 4 and the laser luminous flux scattered is collected on the same circumference on the focal plane for each scattering angle. The detector 6 is constituted by semiconductor photoelectric devices arranged on concentric circles the center of which is on the surface of the focal plane to which front scattering (non-scattering) light of the laser luminous flux is applied. This constitution makes it possible to measure the dependence of intensity of the laser light scattered by the particles in the aerosol on the scattering angles. Here, by utilizing that the dependence of intensity of the scattered laser light on the scattering angles depends on the particle distribution of the group of particles, the particle distribution of the group of particles is calculated by a signal processing device 7.
However, since a visible laser is used as the probe light in the related art, a minimum measurable particle size is about 100 nm. This is because if particles to be measured become smaller in size with respect to a probe light wavelength, in particular, smaller than one tenth of the wavelength, they produce Rayleigh scattering in which the dependence of scattering phenomenon on the particle size is hard to observe and thus the particle size distribution can not be calculated by a scattered light intensity distribution. The use of the fourth harmonic of a Nd:YAG laser can produce ultraviolet coherent light (wavelength: 266 nm) by a comparatively small sized apparatus but, even by this light, a minimum measurable particle size is about 40 nm. In order to produce the ultraviolet light having a smaller wavelength, an excimer laser needs to be used, which in turn increases the size of a light source unit and further limits the use of a transmission type lens in an optical system. To realize the ultraviolet light having a smaller wavelength, it is thought to use ultraviolet light having a wavelength of 126 nm, produced by an Ar2 excimer laser, but even if this ultraviolet coherent light is used, a minimum measurable particle size is about 20 nm. On the other hand, a practical design rule in the semiconductor integrated circuit manufacturing technology is 130 nm at present and will be 70 nm in the year of 2008. Further, generally, it is said that a particle size needs to be controlled within a range of one fifth to the design rule. Therefore, it is impossible to control the particles in the semiconductor integrated circuit manufacturing system to keep and improve a manufacturing yield by using the Rayleigh scattering method described above.